Medic is my most played class.
Pyros are one of the medic's worst enemies. The blutsager (leech needle gun) and back pedaling means I have a decent chance surviving or killing a single pyro.
Spies? I got an ubersaw and not afraid to use it.
Demomen thought.. HATE THEM. The nades mean you can die even if you are at maximum heal range around a corner, while the demoman is sitting by a dispenser 200 feet away.
Edit: Pyros are the reason the team needs one single heavy as a deterrent. One of my greatest pleasures playing as a heavy is not the killing. It's seeing all the enemy pyros scatter and runaway until they get reinforcements once I get into their LOS. RESPECT THE SAMVICH.